Chapter 2: Fundamentals of Data and Signals

In chapter two, there are three main parts. The first part discusses what data and signals are, the second moves on to explain how data can be converted into signals, and finally the text continues to describe the different data codes that are used when transmitting data.

Introduction:

The text first explains the four different combinations of data and signals. 1)Analog data can be converted into analog signal. The most commons devices that perform this technique are radio tuners and TV tuners. 2) Digital data can be converted into digital signals. The most common devices that perform this kind of technique are digital encoders. Local area networks and telephone systems will use this kind of device. 3)Digital data can be converted into analog signals. Modems will perform this kind of conversion. Dial-up internet access, DSL, and cable modems all would use these conversion techniques. 4)Finally, Analog data can be converted into digital signals. A common device that uses this technique is a codec. Music systems will perform these kinds of conversions.

Data and Signals:

This section will explain the differences between digital data and signals from analog data and signals. Data are "entities that convey meaning within a computer or computer systems" (White, 29). Signals are "the electric or electromagnetic impulses used to encode and transmit data" (White, 30). Both data and signals can "exist in either analog or digital form." Analog data and analog signals are continuous wave forms that can contain an infinite number of points between some given minimum and maximum. The minimum and maximum values are represented as voltages. The text provides an example from the human voice. When individuals talk into a telephone, the receiver will convert the airwaves into analog data. The downfall of analog data and signals is "noise" interference. If someone were to speak into the telephone receiver, while someone else is talking right next the individual, the receiver has the possibility of picking up that extra set of airwaves which the receiver will convert into analog data. The telephone system will then send the analog data through transmission of analog signals, which will also contain the noise interference. When data is converted into analog form, it becomes very difficult to separate the noise from the original data. Digital data and signals have a different approach. Digital data and digital signals are created by sets of discrete or fixed numbers of value. Digital data takes on the form of binary 1s and 0s. Digital signals are different. Digital signals create two separate forms. The first form is called "square wave." These patterns contain simple high and low voltage values. The second form contains the combination of "modulate analog signals." In this kind of form, the "noise" that is introduced into the digital signal can be separated easily.

White moves on to explain the three basic components of analog and digital signals. These components are amplitude, frequency, and phase. Amplitude of a signal is "the height of the wave above or below a given reference point." Frequency is the number of times a signal will make a complete cycle within the time frame. Phase is the position of the waveform relative to a given moment of time. The text explains that signals have a tendency to lose power due to friction. This loss of power or signal strength is called attenuation. To measure the attenuation of a signal, we use decibels(dB).

Converting Data into Signals:

This section explains the conversion of data into signals and reminds the reader the four main combinations of data and signals which are: analog data transmitted using analog signals, digital data transmitted using square-wave digital signals, digital data transmitted using discrete analog signals, and analog data transmitted using digital signals.

To transmit analog data with analog signals, we use the process of modulation. Modulation is the process of sending data over a signal by varying its amplitude, frequency, or phase.

Transmitting digital data into square-wave digital signals, we would use the nonreturn to zero digital encoding schemes, manchester digital encoding schemes, bipolar-AMI encoding scheme, or the 4B/5B digital encoding scheme. There are two schemes for the nonreturn to zero digital encoding scheme. The first is the nonreturn to zero-level (NRZ-L), which transmits 1s as zero voltages and 0s as positive voltages. The second is the nonreturn to zero inverted (NRZI), which will transmit at the beginning of 1 as a voltage and will transmit a no voltage at the beginning of a 0. The problem with these two schemes is a synchronization problem. If there is a long sequence of 0s, it would be hard to read because the signal would never change. The Manchester scheme contains two different schemes: the manchester encoding scheme and the differential manchester encoding scheme. The manchester encoding scheme will transfer 1s from low to high voltages in the middle of an interval and will transmit 0s from high to low voltages in the middle of an interval. The differential manchester scheme is similar to the manchester scheme, but instead of changing voltages in the middle of an interval, the voltage will change from the beginning. The next scheme is the Bipolar-AMI encoding scheme. This scheme will transmit voltages on three levels. For 0s, there will be zero voltage and for 1s there can be positive voltages or negative voltages. If the 1 was transmitted as a positive, the next 1 will be transmitted as a negative. 4B/5B digital encoding scheme is the last scheme for transferring digital data with square-wave digital signals. This scheme takes 4bits of data and converts the data into a unique 5-bit sequence. The 5-bits sequence is then encoded using the NRZI scheme. This helps with the baud rate and provides efficiency. The only issue is the extra bit.

There are three ways to transmitting digital data with discrete analog signals. The first is amplitude shift keying. Low amplitudes can represent 1s and higher amplitudes could represent 0s. The problem with this technique is it can be susceptible to sudden noise. The next technique is the frequency shift keying. This approach will use two different frequency ranges to represent the 1s and 0s. Lower frequency signals can be 1s and higher frequency signals can 0s. The downfall for this approach is intermodulation distortion, which is when frequencies of two or more signals mix together and create new frequencies. The final approach is the phase shift keying. The changes in the phase of the waveforms can represent the 1s and 0s. If there was no phase change, that section could be a 0. If there was a phase change of 180 degrees, that section could be a 1.

To transmit analog data with digital signals we could use the pulse code modulation or the delta modulation. Pulse code modulation (PCM) will track the analog wave form and take snapshots of the analog data at fixed intervals. While taking the snapshot, the height or voltage will be calculated for the waveform. This calculation will be converted into a binary value. Delta modulation will track the analog data by assessing the waveforms in up or down steps. For each time period, the codec will decide whether the waveform has risen or fallen. If the waveform drops, it will be represented by a 0. If the waveform rises, it will represent a 1.

Data Codes:

The text explains that there are three important data codes. Those data codes are EBCDIC, ASCII, and Unicode. Extended binary coded decimal interchange code (EBCDIC) is an 8-bit code that will allow 256 combinations of textual symbols. The American standard code for information interchange (ASCII) is a 7-bit version that will allow 128 possible combinations of textual symbols. This data code is governed by the United States and is actually the most widely used data code in the world. Unicode can provide unique coding values for every character in any language.